From Saint-Nom to Poissy through the forests of Marly and Tailles d'Herbelay

Starting point and access: Saint-Nom-la-Bretèche station - Marly Forest.
- Transilien - Line L, terminus of the section between Paris Saint-Lazare and Saint-Nom-la-Bretèche.
Take the only exit at the front of the train.

Red and white markings

(S) At the end of the platform, turn left to take the level crossing. Go up the road and pass a small car park on the left. Take the bridge over the railway line. Turn immediately right and take a path that leads into the woods. Stay on the main path and ignore any side paths. You will come to a crossroads, behind which, a few dozen metres away, you can see a large intersection.

(1) Turn left and you will soon reach another crossroads (signposted "Étoile de la Maison Rouge"): go straight ahead. Heading west or west-southwest, climb steadily up the path, which gradually widens. At each crossroads, go straight ahead.

(2) At the top, at a crossroads (Étoile de Dryade), take the first path on the right. At the next crossroads, near a road (plot no. 106), turn right. Stay on the main path, which curves to the right. You will reach the Croix Saint-Michel.

(3) Then descend to the left on a narrow path that starts at the foot of the cross and two small wooden posts (one on the ground in June 2020). At the crossroads that immediately appears, take the middle path (marked). Go straight ahead heading north, downhill and then uphill. Cross a cycle path and then the D98 ( very busy road) and continue along the path opposite.

No markings

At the first crossroads, turn left (let the^GR®1continue straight ahead). At the next junction, go straight ahead. You will come to a path; follow it to the left for a few metres until you reach a crossroads at the boundaries of plots 10A, 10B and 13 (at this crossroads, some of the paths shown on the map do not exist... or no longer exist).

(4) Then take a path to the north-west that starts just to the right of the tree marked 10A. Further on, you will see the "horseshoe" symbol for a bridle path on the right-hand side. At the next crossroads (elevation 181, signpost), continue straight ahead on Route Hardouin. At the crossroads in a small clearing, continue straight ahead on a wide path. Walk along a pond on your right.

At the next junction (Étoile du Gros Érable; plot no. 23, blue signs with two sides), take the path opposite. Immediately cross a wide grassy track and continue along the path, which appears to be poorly maintained but is well marked. Walk alongside a beautiful cleared forest on your left, then continue through denser undergrowth.

(5) You will come to a small road (with no traffic) and follow it to the right. At the next crossroads (elevation 173), leave the tarmac road, go down the path opposite and you will soon reach a crossroads at the boundaries of plots nos. 23, 35 and 51. Then turn left and walk along a fenced plot on your left.

Red and white markings

At the crossroads (Étoile du Chêne de Retz; bench), turn right, ignore a path on the right and follow a trail, leaving a tree marked with the number 36 on your right. Winding your way along, you will come to a crossroads with large beech trees.

Yellow markings

(6) Then turn right (let the^GR®1continue straight ahead). Ignore a vague turn-off on the right and go downhill. At the crossroads (walls, boundary marker), go straight ahead between two fences and walk alongside a golf course on your right. Go past the entrance gate to the Désert de Retz and walk alongside a wall on your right. Return to the tarmac road, turn right at the roundabout and walk alongside the long building of the Ferme de Retz on your right.

No markings

(7) At Chemin des Jongleurs, leave the tarmac road and take a sharp left (chain) onto a path that enters the Forêt départementale des Tailles d'Herbelay. Walk steadily uphill along a fence on the left and ignore any tracks on the right. At a fork, follow the path on the right, which continues more or less flat. Pass at the foot of a boundary marker on the right-hand side and you will come to a crossroads.

(8) Then take the^second path on the right (north-west; pass a picnic table on your right). At the next crossroads, go straight ahead. Pass a forest barrier and descend. At the crossroads (boundary marker), turn right. On your left, walk alternately along garden fences or a hedge. Keep heading north-east and ignore all paths on the right.

You will come to a road. Cross it at the pedestrian crossing and follow it to the right onto a path protected by a railing and reserved for pedestrians and cyclists. Enter Chambourcy. Go around the roundabout on the left, cross Chemin de Saint-Barthélémy, take the next street (not signposted here) and walk alongside a fenced reservoir on your left.

Yellow markings

(9) At the corner of the fence (EDF power station), turn left onto a path, which is wide and grassy at first, then narrows. Go through a metal gate, continue downhill on the tarmac and ignore all the paths on the left. At the bottom, turn right. Cross a dead end on the left and continue for about 50 metres. When the street turns left, continue straight ahead onto a cobbled street, the Grande Rue. Continue straight ahead until you reach the church.

Red and yellow markings

(10) Then go down a staircase on the left. Cross an esplanade, leave the town hall on your right and go down another staircase. At the bottom, turn right and go down a tarmac path. At the stop sign, turn left. At the roundabout, continue straight ahead. At the end of the street, turn right towards Poissy and go straight ahead until you reach the traffic lights.

Cross the D113 at two successive traffic lights and continue straight ahead on the right-hand pavement. At the stop sign, go straight ahead. Pass by the foot of the Croix de la Laye and take the bridge over the A14 motorway. At the sign for the entrance to Poissy, turn right and enter the Saint-Germain-en-Laye national forest.

No markings

(11) After about 50 metres, at a three-way junction, take the left path (leaving the^GRP®). Immediately turn right onto this path and ignore a path leading left towards a wall. At the crossroads (plot no. 231), turn left. At the next crossroads, take a path on the right.

Red and yellow markings

At the end, rejoin the^GRP® and follow it to the left. Leave the forest (sign with map), go down a staircase, cross Rue Beauregard and make a first turn left then right to follow Rue du Docteur Labarrière. Cross Rue des Grands Champs and continue straight ahead.

No markings

(12) At the corner of No. 16, turn left onto Petite Sente des Culezets (be careful to spot this turn). The path, which is very narrow, first turns right then left and leads to a street: follow this street to the right and downhill. At the end, first turn left then right to follow the Petite Sente des Culezets again, which is much wider here. At the end, turn left and walk along the cemetery wall. You will come to a crossroads at the corner of the cemetery (Poissy prison opposite on the right).

Red and yellow + red markings (very rare)

(13) Cross the crossroads, turn left then right, and go up Avenue Blanche de Castille on the left. Take the^first right, Avenue Christine de Pisan. At the end, turn right and follow Enclos de l'Abbaye. After a right turn and then a left turn, go under a porch. Further on, at the Toy Museum, go under another porch.

You will come out onto an avenue; follow it to the left. Cross at the first pedestrian crossing, at Rue de la Caserne, and turn left again. Take the^first right, Rue de l'Église, to reach the south entrance of the Collégiale Notre-Dame.

(14) Retrace your steps and, at the corner of the bell tower porch, turn right and cross a tree-lined square. At the end, first turn left then right to cross Rue Saint-Louis at the pedestrian crossing. Continue straight ahead on Avenue Meissonier. Go under the railway bridge and turn right onto Cours du 14 Juillet. Walk along the Seine to the Old Bridge of Poissy (remains on the left).

Ignore Rue de la Gare on the right and continue for a few dozen metres along the tree-lined bank. Then take the pedestrian crossing on the right, which allows you to cross a wide avenue in two stages. Leave the bus station on your left, turn right and head for Poissy station (E).

To return home:
- RER - Line A, towards La Défense and Paris.
- Transilien - Line J, towards Mantes-la-Jolie or Paris-Saint-Lazare.

Train timetables: consult the Transilien website.

For motorists:
Park at least one vehicle in the car park of each of the two stations, which are not on the same line. Travelling with a single vehicle would require a connection at La Défense Grande Arche or Paris Saint-Lazare.

Hiking boots are recommended.

No water sources identified along the route. Bars, restaurants and shops in the town centres of Chambourcy, before (10), and Poissy, between (14) and (E).

Detailed map required (at least the one accompanying this description).

Warning:
Hikers should be aware that the route passes close to the A13 motorway and that the heavy traffic there is bound to be audible.

Hike completed by the author on 21 September 2019.

From (S) to (9), in the forest:
- Pleasant trails.
- Some remarkable trees.
- Croix Saint-Michel (18th century).

(10) In Chambourcy:
- Sainte-Clotile Church (17th-19th century).

From (14) to (E), in Poissy:
- Notre-Dame Collegiate Church (11th-16th century).
- Remains of the Old Bridge of Poissy (12th century).